Fiber reinforced polymer composite parts are useful in many applications and offer weight savings as parts for automotive vehicles. For example, vehicle body panels have been compression molded of glass fiber reinforced sheet molding compound (SMC). In the case of automotive external body panels they must be painted to have a commercially acceptable glossy appearance free of surface and edge defects.
The term “polymer composite” broadly refers to polymer based compositions that are formulated to contain additives to improve their properties for a specific application. The polymer composites may contain, for example, reinforcing fibers, fillers, pigments and other polymers. In the case of automotive vehicle body panel applications, polymer composites include, for example, compression molded sheet molding compound (SMC) containing unsaturated polyester and polystyrene resins, reinforced reaction injection molded (RRIM) polyurea polymers, or injection molded products containing poly (phenylene oxide) (PPO)/nylon based resins. Such polymer composite parts are lighter than comparably sized steel panels. However, the composites do have to be painted for body panel applications, and it has been difficult to paint the composite body panels without introducing surface defects.
Automotive painting operations are typically carried out on a body-in-white. A body-in-white is the unpainted unitary body structure comprising body panels and structural components. Such a body structure is usually formed mostly of steel panels but now may include some polymer composite panels. The paint shop practice is established for the steel portion of the body which is electrically conductive and receives several coating layers for corrosion resistance, paint adhesion and painted surface finish quality. The composite panels do not respond to the several coating procedures in the same way as the steel panels. For example, automotive painting operations often involve the separate application of a zinc phosphate base layer, an electrocoated liquid prime coat using water or an organic solvent, a liquid or powder primer surfacer layer, a liquid base color coat and a liquid or powder clear top coat. But there is no deposition of either the zinc phosphate coating or the electrocoated prime on the typical composite panel surfaces.
Following each of the prime coat, the primer surfacer and the clear top coat applications there is a baking step at temperatures of 250° F. or higher to cure or dry the new layer and to promote flow of the top coat films to a commercially acceptable finish for a vehicle. Such aggressive heating of the painted composites typically leads to “out-gassing.” Out-gassing is the release of entrapped air, solvent, moisture, and uncured chemicals and polymer precursor materials from the somewhat porous composite substrate. Too often the result is an unsightly and unacceptable rough surface. Out-gassing was initially experienced with liquid primer surfacer paints at their 250° F. bake temperature. The occurrence of surface roughness with such paint systems has been reduced in some instances by the use of a special formulated, electrically conductive polymer prime coat as a barrier coat after molding. This polymeric prime coat on the composite surface may reduce out-gassing at that location. But this coating doesn't appear to work for all molded polymer composite and liquid paint combinations, and it completely fails to prevent out-gassing during the flow and curing of powder paints which require even higher bake temperatures (350° F.).
U.S. Patent Application publication 2003/0201186 A1, entitled Metallization of Polymer Composite Parts for Painting and U.S. patent application Ser. No. 10/304,086, filed Nov. 25, 2002, entitled Metallization of Polymer Parts for Painting, both assigned to the assignee of this invention, disclose methods of treating the surfaces of polymer composite articles of manufacture to avoid out-gassing caused defects during post-molding painting operations. Those patent specifications disclose the practice of providing a conductive metal coating on molded polymer composite surfaces to permit, for example, the phosphating and subsequent prime coatings and top coatings of automotive body panels yielding uniform appearing and high quality surface finishes. They are methods that result in the formation of, for example, a zinc or zinc based alloy coating on the composite surface prior to painting. Other metals such as aluminum or iron compositions may be suitable, but zinc is preferred. The zinc coating prepares the surface of the composite part for phosphating or the like, if desired. The zinc layer makes the surface of the composite conductive for electrostatic painting with liquid (solvent or water based) or dry powder paints, and it provides an impermeable layer to prevent out-gassing from the composite into paint layers, especially during paint drying or curing steps. The entire texts of the above identified patent specifications are incorporated herein by reference.
Sometimes it is preferred to etch the surface of a molded composite polymer article to receive and anchor a coating of suitable metal. Acid etching attacks the cross-linked polymer matrix and roughens and oxidizes the surface of the heterogeneous composite composition. Deposited metal can then interlock with the cratered surface. In the case of a fiber reinforced polymer composite article, etching exposes but does not remove the reinforcing fibers (for example, glass or carbon fibers) and the exposed fibers provide an undesirable surface for the deposition of a metal layer. It is an object of the present invention to provide a process for altering the surface of a molded fiber reinforced polymer composite article to better receive and hold an adherent coating of metal preparatory to painting of the article.